Rotary seals are placed between two parts, one or both of which may rotate. Rotary seals may be used to seal the air system of a gas turbine engine.
Measurement of seal leakage flow in rotary seals is particularly difficult due to the multitude of inlet and outlet flow paths on each side of the seal.
Measurement of mechanical clearance of seal parts may be used to obtain information relating to the seal leakage flow, however such measurements have the disadvantage of requiring bulky probes to be introduced into the gas turbine engine. The rotary seal parts are unsuitable for mounting such probes and so the probes need to be mounted on nearby components which support the seal, introducing uncertainty into the clearance measurement due to the mechanical behavior of the components.